3D time-of-flight (ToF) cameras acquire depth images by determining the time which radiation, preferably light, needs from a source to an object and back to the camera. This is often done by illuminating the scene discontinuously and applying a convolution of a temporal window (strictly speaking: a sequence of windows) to the backscattered incident optical signal. Continuous-wave ToF cameras illuminate the scene using a periodically modulated light-source, and measure the phase shift of the backscattered signal relative to the emitted signal. This phase shift is proportional to the time-of-flight, so it contains the distance information. Typically, three quantities are unknown and have to be determined for each pixel individually: the object's distance, its reflectivity and the intensity of ambient light. Therefore, one or more (dependent on the number of unknowns) measurements, for instance at least three measurements in case of three unknowns, are necessary to determine these unknowns.
A Continuous-wave ToF sensor (PMD sensor) is described in Schwarte, R., Heinol, H. G., Xu, Z., Hartmann, K.: New active 3D vision system based on rf-modulation interferometry of incoherent light, in Casasent, D. P. (ed.) Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, vol. 2588, pp. 126-134 (1995) and in Spirig, T., Seitz, P., Heitger, F.: The lock-in CCD. Two-dimensional synchronous detection of light. IEEE J. Quantum Electronics 31, 1705-1708 (1995).
More information about the general technology of ToF cameras can be found in Elkhalili, O., Schrey, O., Ulfig, W., Brockherde, W., Hosticka, B. J., Mengel, P., Listl, L.: A 64×8 pixel 3-D CMOS time-of flight image sensor for car safety applications (2006), in Gokturk, S. B., Yalcin, H., Bamji, C.: A time-of-flight depth sensor—System description, issues and solutions, in http://www.canesta.com/assets/pdf/technicalpapers/CVPR_Submission_TOF.pdf, and in Oggier, T., Lehmann, M., Kaufmann, R., Schweizer, M., Richter, M., Metzler, P., Lang, G., Lustenberger, F., Blanc, N.: An all-solid-state optical range camera for 3D real-time imaging with sub-centimeter depth resolution (2004), Proceedings of SPIE 2003, pp. 534-545, 2003, and in Ringbeck, T., Hagebeuker, B.: A 3D time-of-flight camera for object detection, Optical 3-D Measurement Techniques 09-12.07.2007 ETH Zurich, Plenary Session 1: Range Imaging I.
Many known TOF cameras (e.g. as described in the above cited disclosure of Ringbeck, T. et al.) use a special sensor, which employs two quantum wells per pixel to measure the correlation function of a detection signal representing the detected radiation (in particular light) with an electronic reference signal. Incident photons generate electrons, they are sorted by a switch into these two quantum wells, converted into a voltage, amplified, and given out as two digital values (also called “samples”). The switch is synchronized with the light source, thus the two digital values correspond to two samples of the correlation function shifted by 180° against each other. By delaying the reference signal by a certain angle Θ, the sensor is able to sample arbitrary points of the correlation function. Typically, Θ is chosen as {0°, 90°, 180°, 270°}, the data acquired by both quantum wells A and B correspond to Θ and Θ+180°, respectively. This gives eight samples A—0, A—90, A—180, A—270 and simultaneously acquired B—180, B—270, B—0, B—90. Thus, each point Θ is sampled twice (e.g. A—0 and B—0). The reason for that is the need for compensating sensor inhomogeneities since the quantum wells A and B and their amplification paths do not respond equally to radiation. An averaging of the eight samples is then used to obtain one value of the phase shift and therefrom one value of the distance.